1. Field of the Invention
The present invention generally relates to a positive electrode current collector for an alkali metal, solid cathode, nonaqueous liquid electrolyte electrochemical cell, and more specifically to cobalt-based alloys as positive electrode current collector materials.
2. Prior Art
Solid cathode, liquid organic electrolyte, alkali metal anode electrochemical cells or batteries are used in applications ranging from power sources for implantable medical devices to down-hole instrumentation in oil/gas well drilling. Typically, the battery is comprised of a casing housing a positive electrode comprised of cathode active material, material to enhance conductivity, a binder material, and a current collector material; a negative electrode comprised of active material such as an alkali metal and a current collector material; a nonaqueous electrolyte solution which includes an alkali metal salt and an organic solvent system; and a separator material encapsulating either or both of the electrodes. Such a battery is described in greater detail in U.S. Pat. No. 4,830,940 to Keister et al., which is assigned to the assignee of the present invention and incorporated herein by reference.
The positive electrode current collector serves several functions. First, the positive electrode current collector acts as a support matrix for the cathode material utilized in the cell. Secondly, the positive electrode current collector serves to conduct the flow of electrons between the active material and the positive cell terminal. Consequently, the material selected as the positive electrode current collector affects the longevity and performance of the electrochemical cell into which it is fabricated. Accordingly, the positive electrode current collector material must maintain chemical stability and mechanical integrity in corrosive electrolytes throughout the anticipated useful life of the cell. In addition, as applications become more demanding on electrochemical cells containing nonaqueous electrolytes (including increased shelf life and extended long term performance), the availability of corrosion resistant materials that are suitable for these applications becomes more limited. For example, the availability of materials capable of operating or maintaining chemical stability at elevated temperatures is limited. Elevated temperatures may be encountered either during storage or under operating conditions (elevated temperature discharge down-hole in well drilling), or during autoclave sterilization of an implantable medical device powered by the electrochemical cell (Thiebolt III and Takeuchi, 1989, Progress in Batteries & Solar Cells 8:122-125).
The prior art has developed various corrosion resistant materials useful for positive electrode current collectors. However, certain materials corrode when exposed to elevated temperatures of about 72.degree. C. or higher or when exposed to operating conditions in aggressive cell environments wherein surface passivity is compromised. Also, at elevated temperatures the chemical integrity of the positive electrode current collector material may depend on the cathode active material incorporated into the cathode. For example, if titanium is used as the current collector material and the cathode active material is fluorinated carbon, titanium can react with species present within the internal cell environment to undesirably increase cell impedance by fluorination and excessive passivation of the current collector interface (Fateev, S. A., Denisova, O. O., I. P. Monakhova et al., Zashchita Metallov, Vol. 24, No. 2, pp. 284-287, 1988, transl.). The kinetics of this process are temperature dependent. At elevated temperatures, excessive passivation may occur quite rapidly (for example, at 100.degree. C., the reaction requires less than 10 days).
Other current collector alloys used to fabricate positive electrode current collectors have been described in the art. Highly alloyed chromium-containing stainless steel materials are described in Japanese patent publications Nos. 18647 and 15067. However, the ferritic stainless steel material disclosed in publication No. 15067 requires costly melting procedures, such as vacuum melting, to limit the alloy to the cited carbon and nitrogen levels. Highly alloyed nickel-containing ferritic stainless steel materials, which provide superior corrosion resistance, particularly where elevated temperature storage and performance is required, are disclosed in U.S. Pat. No. 5,114,810 to Frysz, et al., which patent is assigned to the assignee of the present invention and incorporated herein by reference. However, use of such alloyed ferritic stainless steels is limited in several respects. Chief among them is the alloy is not readily available in thicknesses typically required for use as a current collector, and developing a commercial source has proven difficult. Current collectors are preferably thin to permit increased volumetric and gravimetric energy density, as well as to permit increased surface area per volume for rapid discharge at high current densities.
Therefore, the present invention is directed to providing a positive electrode current collector material which exhibits chemical compatibility with aggressive cell environments; provides high corrosion resistance but does not develop excessive passivation in the presence of fluorinated materials such as fluorinated carbon materials, and thereby maintains its inherent high interfacial conductivity; provides resistance to surface activation by material handling or mechanical means; and is manufacturable in the required form and thicknesses.
Cobalt-based alloys according to the present invention offer the characteristics required of such positive current collectors. This class of metals also offers other advantages, especially when used in cells for implantable medical devices. Typically, the power source of an implantable medical device contains current collectors made from wrought metal stock in sheet or foil form by convenient and economical chemical milling/photoetching processes. The present cobalt-based alloy current collectors are readily fabricated by these processes in contrast to the prior art high chromium ferritic alloys. The latter materials are generally formed by mechanical punching/expansion techniques which tend to leave sharp burrs on the current collector. It is costly to deburr such components and the burring condition limits collector configurations.
Even in the family of cobalt-based alloys, however, selection is limited. It is known to developers of cobalt-based alloys that certain elemental constituents, especially chromium, molybdenum and tungsten, are of vital importance in maximizing corrosion resistance. Thus, the total amount of chromium, molybdenum and/or tungsten present in a particular cobalt-based alloy is a primary determinant to the suitability of that alloy as a current collector. For example, HAVAR.TM., a cobalt-based alloy commercially available from Hamilton Precision Metals, Inc., Lancaster, Pa., has by weight percent, 42% cobalt, 19.5% chromium, 12.7% nickel, 2.7% tungsten, 2.2% molybdenum, 1.6% manganese, 0.2% carbon, with the balance being iron. HAVAR.TM. has a combined chromium, molybdenum and tungsten content of about 24.4 weight percent and readily corrodes in certain cell environments in which ELGILOY.RTM., typically containing a total of about 27 weight percent chromium and molybdenum, does not corrode. HAYNES.RTM. Alloy 556 containing a total of about 30 weight percent, chromium, molybdenum and tungsten also does not corrode. Consequently, there are only a handful of acceptable compositions among available metals and alloys which remain practically corrosion-free in certain demanding cell environments; high chromium ferritic stainless steels are one class and selected cobalt-based alloys are another.